Xerographic process utilizing a photoconductive alloy of thallium in selenium



United States Patent ()fitiee 3,427,157 Patented Feb. 11, 1969 3,427,157 XEROGRAPHIC PROCESS UTILIZING A PHDTOCONDUCTIVE ALLOY OF THAI.- LIUM HQ SELENIUM Peter J. Cerlon, Rochester, N.Y., assignor to Xerox Corporation, Rochester, N.Y., a corporation of New York No Drawing. Filed Dec. 28, 1964, Ser. No. 421,626 U.S. Cl. 96-1.4 7 Claims Int. Cl. (203g 13/22, 7/00 ABSTRACT OF THE DISCLOSURE A photoconductive alloy comprising an amorphous film of from about 0.0001 percent by weight to about percent by weight of thallium in selenium.

This invention relates to a photoconductive alloy, xerographic plates made from the alloy and methods for the use of such plates.

Heightened interest has recently developed in photoconductive alloys which are particularly adapted for specific end uses. This interest has, of course, largely been generated by the recent market growth for television pick-up tubes, office copying machines and other devices, making use of these photoconductive alloys.

Xerographic office copying, for example, has undergone an extremely large growth in the past few years. In this copying technique, as originally disclosed by Carlson in US. Patent 2,297,691 and as further amplified by many related patents in the filed, a photoconductive insulating layer making up part of a xerographic plate is first given a uniform electrostatic charge over its entire surface to sensitize it and is then exposed to an image of activating electromagnetic radiation such as light, X-ray, or the like which selectively dissipates the charge in illuminated areas of the photoconductive insulator, leaving behind charge in the non-illuminated areas to form a latent electrostatic image. This latent image is then developed or made visible by the deposition of finely divided, electroscopic marking material on the surface of the photoconductive insulating layer as a result of which the marking material conforms to the pattern of the latent image. Where the photoconductive insulating material is reusable, this visible image of finely divided or powdered marking material is then transferred to a second surface, such as a sheet of paper, and fixed in place thereon to form a permanent visible reproduction of the original. Where, on the other hand, a cheaper, nonreusable, photoconductive insulating material is employed, the toner particles are fixed in place directly on its surface with the consequent elimination of the transfer step from the process.

Although high-quality xerographic plates made with photoconductors, such as the amorphous form of elemental selenium, as described in US. Patent 2,970,906, to Bixby, have been used in the basic form of xerography described above with great success because of their inherent properties, these same properties make them unsuited to other imaging processes. Thus, for example, since amorphous selenium responds to bot-h the visible light and X-ray portion of the electromagnetic spectrum, plates made from this material must be very carefully handled and protected from ambient light when they are used for X-ray imaging. In addition, because of the very fact that amorphous selenium xerographic plates can be recycled through the conventional xerographic copying process time after time in rapid succession without prior image exposures in previous cycles having any effect on the photoconductive insulating response of the selenium in successive cycles, this material is unsuited to duplicating processes which depend for their efficacy upon the retention of light pattern-induced conductivity patterns in the photoconductor during successive reproduction cycles. Such a photoconductor, which may be referred to as a high fatigue photoconductor, need only be exposed to a light image or other electromagnetic radiation pattern once and then a great number of duplicates of this image can be formed by reprocessing it through the cycle which includes uniformly applying charge to its surface, developing it, and transferring the developed image to a copy sheet. Because it is a high fatigue material which remembers the exposure pattern to which it was initially subjected, it remains relatively conductive in those areas which were initially exposed to the image and the re-application of uniform charge during each duplicating cycle only results in a recharging of the background areas to the exclusion of the initially exposed areas which rapidly dissipate this charge after each charging step.

Accordingly, it is an objective of this invention to define a novel photoconductive alloy.

It is a further object of this invention to describe a photoconductive alloy which may be easily deposited in very smooth, thin layers.

A further object of this ivention is to define a novel xerographic plate whose properties may be varied according to the polarity of charge which is applied.

Still another object of this invention is to define a novel xerographic plate which is highly sensitive to X- ray exposure and substantially completely insensitive to exposure to visible light.

It is also an object of this invention to define a novel high fatigue xerographic plate which is especially well suited to duplicating.

The above and still further objects may be accomplished in accordance with the present invention, generally speaking, by employing a photoconductive insulating alloy of selenium and thallium not only as a generally useful photoconductive insulating material but also, more specifically, in a xerographic plate. This type of xerographic plate is especially well adapted to X-ray xerography, hereinafter referred to as xeroradiography when positive charging is employed to sensitize it and to fatigue duplicating when negative charge is employed to sensitize it.

As stated above, the photoconductor in this invention is made by alloying thallium with selenium in its amorphous form. Although up to about 5% by weight of thallium may be employed in the alloy, it is to be noted that the amorphous selenium is extremely sensitive to thallium and additions of thallium as small as one part per million (0,0001%) have shown marked effects on the xerographic properties of the alloy. A preferred range of thallium percentages by weight to be included within the alloy is from about 0.0005% to about 0.0020% because higher additions tend to contaminate the coater and apparently do not have additional beneficial eifects. The thallium may be added to the selenium by any suitable technique. Thus, for example, the selenium may be heated above its 'melting point followed by slow addition of the required amount of thallium to the molten selenium with stirring to achieve a good uniform mixing of these two components. It should be made clear at this point that it is not certain whether or not the thallium which is blended into the selenium actually undergoes a reaction with the selenium to form a small amount of a thallium selenide compound in the excess selenium or whether a solution of the two materials is formed. Accordingly, it is to be understood that the terms mixture and alloy are to be read in their broadest sense throughout this specification and the appended claims to encompass both selenium with a small amount of the thallium selenide compound blended or dissolved therein and a mere blend of the two unreacted elements, thallium and selenium, although it is believed that the compound is formed. The alloy is deposited in its amorphous form which appears very similar to the unalloyed variety of selenium in its amorphous form. It is to be understood that when the amorphous form is referred to throughout the specification and claims, it is also intended that the amorphous layer with small amounts of crystallites or crystal nuclei dispersed throughout shall be included within the purview of this term.

Other alloying elements or compounds may also be added to the photoconductive alloy of this invention to vary its spectral response, increase its crystallization resistance, etc.

For many purposes, it may be desirable to coat the alloy on a supporting substrate to give additional strength, contact with a source of electrical potential, etc. Thus, for example, in using the alloy in a xerographic plate, a layer of it may be coated on a conductive substrate such as aluminum, magnesium, brass, steel, chrome and nonconductive materials such as glass, paper, plastic sheeting and the like coated with conductive coatings such as thin layers of gold, tin oxide, copper iodide or the like. The use of such a conductive substrate not only provides additional structural strength to the xerographic plate but also provides for an electrical ground plane immediately beneath the surface of the alloy layer so that it may be easily charged from a corona discharge electrode in accordance with the teachings of U.S. Patent 2,588,699 to Carlson. It is to be noted that for this purpose, the conductive material need not necessarily be a material which would ordinarily be thought of as an electrical conductor. Any substrate having an electrical resistance, at least several orders of magnitude lower than the resistance of the illuminated photoconductor such as certain glasses and plastic resins, will serve admirably even without metallic coatings. If, on the other hand, certain other charging techniques are employed, such as the two-sided corona charging technique described in U.S. Patent 2,922,883 are employed, the conductivity of the supporting substrate may be largely ignored, and it may be selected based mainly on its structural properties. In the event that the alloy is applied to a substrate, any suitable application technique may be employed so long as the material is frozen in amorphous form without substantial crystallization. Typical coating techniques employed in the art include vacuum evaporation to a cool substrate, casting, dip coating with a doctor blade, etc. Vacuum evaporation is, however, a preferred coating technique because of the extremely smooth and uniform surfaces which it is capable of producing. The thickness to which the photoconductive alloy is coated is, of course, dependent upon the particular end use to which it is to be put. Thus, for example, in some systems, very thin coatings on the order of a few microns may be employed, while for most purposes in conventional xerographic copying, thicknesses of from about 40 to about 100 microns may be employed, and for X-ray imaging or use with other high energy electromagnetic radiation, much thicker layers on the order of from 250 to 500 microns may be employed to take advantage of themcreased stopping power of the thicker layer.

An important advantage of the photoconductive alloy of this invention when used in a xerographic plate is that it has markedly different xerographic properties when charged positively as opposed to those which it has when it is charged negatively. On positive charging, a plate made with the alloy is virtually insensitive to visible light, while retaining a very high degree of sensitivity to the X-ray portion of the electromagnetic spectrum (about 10 to 10 cm. wavelength). On the other hand, when the plate is negatively charged, it is highly sensitive to visible light (about 4000-7000 Angstroms wavelength) but exhibits an extremely high remanence of fatigue effeet after the first image exposure. Accordingly, after the initial exposure, it may be charged time and time again and will only retain a charge pattern in unexposed areas. This pattern may then be developed after each charging step and transferred to a paper copy sheet so that a great number of copies may be made duplicating the original image without re-exposing the photoconductive material for each cycle of operation.

The general nature of the invention having been set forth, the following examples are now presented in more specific illustration thereof.

EXAMPLE I 99 parts by weight of a high purity grade of selenium (99.99%+ pure) and 1 part by weight of elemental thallium pre-alloyed by heating together under nitrogen, are placed in an inert pyrex evaporation boat in a vacuum evaporation chamber below an aluminum plate with a thin aluminum oxide surface coating thereon. The vacuum evaporation unit is then sealed and evacuated to a vacuum of approximately 10- mm. of mercury with the aluminum plate held at 50-60 C. The pyrex boat is heated up to a temperature of about 280 C. to cause avaporation of the selenium-thallium mixture in a uniform coating by condensation on the aluminum plate surface. Although thallium has a melting point of about 300 C., the thallium goes into solid solution in the molten selenium which melts at about 215 C. and is evaporated across with the selenium. A very uniform, shiny, amorphous film with a thickness of about 160 microns is thus formed. After completion of the evaporation step and cooling, the vacuum is broken and the plate is removed therefrom. The thus formed plate is then charged positively with a corona discharge electrode as described in U.S. Patent 2,777,957 to Walkup and a scan of the plate with an electrometer indicates that it accepts an initial charge voltage of 760 volts. A second electrometer reading made after 3 minutes exposure of the plate surface in ambient room light indicates the plate still is holding over 700 volts of charge. Re-exposure of the same plate with a conventional visible light exposure source also leaves behind substantially all of the initial charge in exposed areas with a residual voltage of about 675 volts according to the electrometer reading. An attempt to develop the thus exposed plate is unsuccessful. The same plate is then cleaned and recharged, reaching about the same 760 volt initial potential, exposed to an X-ray image of a large cow bone and developed with a cascade developing mixture in a developing apparatus described in detail in U.S. Patent 2,600,580 to Sabel to produce a high-quality visible image of the bone from the latent electrostatic image formed by the X-ray exposure. The developer mix comprises a mixture of fine toner particles, as described in U.S. Patent 3,079,342 to Insalaco, and coated carrier beads, as described in U.S. Patent 2,618,551 to form a developer mixture of the type described in U.S. Patent 2,638,416. Once development is completed, the developed toner image is transferred to a paper copy sheet where it is fixed by heat fusion.

. EXAMPLE II The plate of Example I is charged with a corona discharge electrode of the type employed in Example I except that the polarity of the electrode is reversed so as to impart a negative charge to the plate surface. Measurement of the plate potential with an electrometer indicates an initial charge of 675 volts. The plate is then contact exposed with an original on the light source of the apparatus in the aforementioned Sabel patent and developed as in Example I. After transfer of the developed image, the plate is recharged and scanned with an electrometer. Although the plate holds about the same voltage in areas which have not been exposed in the first exposure step, about 98% discharge is found in previously exposed areas. The plate is then developed without any additional exposure and the developed image is transferred to a paper copy sheet where it is fixed, producing a high-quality image of the first original. This processing cycle of negative charging, development and transfer is then carried on more times without noticeable deterioration in image quality of the copies produced.

EXAMPLES III-V 11 Xerographic plates are made up according to the procedure of Example 1 except that the percentages by weight of thallium included in the selenium plates are 0.5% for Example III, 0.1% for Example 'IV, 0.05 for Example V, 0.01% for Example VI, and 0.0001% for Example VII. Each of these plates is tested according to the procedures of Examples I and II with substantially equivalent results except for a gradual decrease (corresponding to the amount of included thallium) in the residual voltage which the plate holds after 3 minutes of visible light exposure. This decrease is, however, largely insignificant in that even the plate of Example VH with 1 part per million of included thallium still holds 500 volts after light exposure on positive charging and shows the same type of X-ray response as the plate with 1% by weight of included thallium.

EXAMPLE VH1 To be sure that other fortuitously included impurities have had no effect on the tested plate properties, a pure amorphous selenium plate is made up from the same batch of selenium used to form the plates of Examples 1 through VI and tested according to the procedure of Examples I and 11. This plate shows no detectable residual voltage after 3 minutes light exposure and no detectable fatigue pattern based on prior exposures when tested according to the procedure of Example What is claimed is:

1. A method of forming a latent image comprising uniformly charging a xerographic plate including a photoconductive insulating layer comprising an amorphous film of an alloy of from about 0.0001% by weight to about 5.0% 'by weight of thallium in selenium and exposing said charged plate to an image with actinic electromagnetic radiation.

2. A method according to claim 1 further including the step of developing said exposed plate with finely divided electroscopic marking particles.

3. A method of zerographic imaging comprising positively charging a xerographic plate including a photoconductive insulating layer comprising an amorphous film of an alloy of from about 0-.0001% by weight to about 5% by weight of thallium in selenium, and exposing said charged plate to an image with radiation from the X-ray portion of the electromagnetic spectrum.

4. A method according to claim 3 further including the step of developing said exposed plate with finely divided, electroscopic marking particles.

5. A method according to claim 3 in which said process steps are carried out during exposure of said plate to ambient light from the visible portion of the electromagnetic spectrum.

6. The method of forming a latent fatigue pattern in a xerographic plate comprising negatively charging a xerographic plate including a photocond'uctive insulating layer which comprises an amorphous film of from about 0.0001% by weight to about 5% by Weight of thallium in selenium and exposing said :charged plate to the desired pattern with radiation from the visible portion of the electromagnetic spectrum.

7. A method of duplicating comprising forming a latent fatigue pattern on a xerographic plate including a photoconductive insulating layer which comprises an amorphous film of from about 0.0001 percent by weight to about 5 percent by weight of thallium in selenium, developing said pattern with finely-divided, electroscopic marking material, transferring said marking material to a copy sheet, and then recycling said xerographic plate to all of the aforementioned process steps except for the exposure step.

References Cited UNITED STATES PATENTS 2,804,396 8/1957 Ullrich 961.5 X 2,862,815 12/1958 Sugarman et a1 961.5 2,887,411 5/1959 Hoppe et al. 117200 2,968,725 '1/ 1961 Ter-Pogossian 117-33.5 X 2,962,376 11/ 1960 Schafiert 961.5 2,970,906 2/1961 Bixby 961.5 3,077,386 2/1963 Blakney et al. 96-15 X 3,312,548 4/ 1967 Stranghan 961.5

FOREIGN PATENTS 898,641 12/ 3 Germany.

NORMAN G. TORCHIN, Primary Examiner. C. E. VAN HORN, Assistant Examiner.

U.S. Cl. X.R. 96-1, 1.5 

